Session 3
“Chemical properties and their effect on productivity”

Most of the arable soils in Northeast Thailand are typical tropical sandy soils.
Their main primary and secondary minerals are quartz and kaolinite, respectively, because
parent materials have been strongly weathered. As a result of the destruction of natural
vegetation to make room for cultivation, the soil organic matter is low resulting in low
cation exchange capacity (CEC) and low pH. Amelioration of these soils requires liming, fertilization
and application of organic matter and 2:1 type clay minerals. Each of these ameliorating techniques
encounters respective problems. Rather many farmers are using animal dung as an organic
fertilizer for cash crops and/or rice seedlings. This practice has some
limitations. Green manure has been considered to be useful, though its extension
has not been successful due to lack of proper techniques of cultivation and utilization of
suitable plants. A part of the arable soils in the region are salt-affected, salinization being intensified by deforestation.
Reforestation is not always effective in desalinizing the salt-affected soils, because the degree of
salinization varies markably according to the position in the relief and both short- and long-term strategies are
needed. This paper discusses laboratory, greenhouse and field strategies
for overcoming these problems in sandy soils of Thailand.

Introduction

Most of the arable soils in Northeast Thailand are
sandy, acidic and infertile. Their primary and secondary minerals are mainly quartz and kaolinite, respectively. This is because their parent
materials are highly weathered.
These infertile soils are liable to be degraded
by human activities. In this sense, these soils can be said to be
typical tropical sandy soils. This paper
discusses the characteristics and amelioration strategies of these soils mainly based on the research results of Agricultural Development
Research Center
in Northeast Thailand (ADRC), a project of Japan International Cooperation Agency (JICA).

Northeast Thailand is a square
shaped plateau almost
completely surrounded by mountain ranges and divided into two basins (Khorat
Basin, Sakon
Nakon Basin) by a relatively small mountain range (Phu Phang Range) (Figure 1). These basins are composed of hilly, undulating and flat
low-lying regions. In the flat low
lying region, large rivers (e.g. Mun River, Chi
River) flow along these mountain ranges. According to Koppen’s
system, the climate belongs to tropical savanna
with an alternation of rainy and dry seasons. In the rainy season,
erratic and small rainfall shows two peaks (Figure 2). In the past, a large part of Northeast Thailand was covered with the forests of Dypterocarpacae
(Boulbet 1982, Ishizuka 1986). Deforestation
has proceeded gradually in old times and rapidly in recent years in parallel with intensification of human
activities (Tasaka 1991).

Figure 1. Landform

The
main crops have been paddy rice and cassava,
though other crops are replacing cassava. The number of cattle is low due to a lack of pasture. Irrigation systems are not well established and a
large part of the arable land, especially the paddy field, remains under rain-fed conditions. Traditionally,
they transplant rice seedlings when the paddy field is sufficiently flooded without fear of drought.
Under these circumstances, most of
the farmers are very poor, eager to
get cash income by working away from home and have not enough experience
and knowledge to utilize high technologies.
Accordingly, ameliorating technologies
acceptable to the farmers must be cheap, simple and profitable. If
technologies are really appealing to farmers
they will adopt them without any effort of extension.

Figure 2. Moisture regime

Nature and properties of the sandy soils Profile

Typical vertical arrangement of
the soil horizons or
layers is shown in Figure 3. The texture of the soil becomes finer with increasing depth. This may be mainly caused by selective erosion of fine
fractions from the surface horizons
(Mitsuchi et al. 1986). Two gravel layers are present between the soil
and 4 substrata. Upper 3 substrata (mottled zone, pallid zone and saprolite
zone) are actually weathered products of the 4th reddish stratum,
which is the uppermost part of Mahasarakham
formation of the Cretaceous to Tertiary period (Mitsuchi et al. 1986, Wada et
al. 1994). The mottled zone and the pallid zone are enriched with 2:1
type clay minerals and CaCO3.
This profile suggests that the parent material of the sandy soil is transported by wind in the past
when Northeast Thailand was significantly
drier than the present. The gravel layers and the saprolite zone with cracks
are an aquifer of shallow unconfined groundwater and an aquifer of deep semi-confined groundwater, respectively
(Khoyama and Subhasaram
1993, Wada et al. 1994).

Figure 3. Vertical arrangement of horizons and layers

Other types of vertical profile
are modifications of
the above mentioned typical vertical profile. One type of the modified profiles is
often found along the big rivers due to strong erosion and sedimentation for a long period.

Some chemical properties of the sandy soils

The arable sandy soils are acid,
poor in organic matter
and macro- and micro-nutrients and low in CEC (Motomura et al. 1979, Ogawa et al. 1980, Bell et
al. 1990, Ishida et al. 1993). The low
organic matter content may be caused by low
ability of the soil to stabilize organic matter as well as by the rapid
decomposition of organic matter under tropical conditions (Wada 1996). Arable soils are remarkably inferior to their corresponding native forest
soils in terms of almost all the
important properties relating to soil
fertility (Ohta et al. 1992, 1996). In other words, deforestation lowers contents of organic matter
and mineral nutrients, CEC and pH.
The main reasons are: (1) Deforestation enhances loss of organic matter,
because supply of organic matter in the form
of litter is curtailed and
decomposition of organic matter is accelerated, (2) CEC decreases in
parallel with the decrease in organic matter content, because CEC is principally attributed to organic matter
(Figure 5); (3) deforestation
destroys the cycling of basic cations like
Ca and K inside the forest by both runoff and leaching that carry away the basic cations which are supplied to
the soil from the subsurface horizons through litter fall. (Figure 6). In addition, (1) main
part of the soil organic matter takes the form of plant debris liable to be decomposed (Wada
1996), (2) the soil is rather easily acidified even to the subsoil by repeated application of chemical
fertilizers, (3) the acid tropical soil contains Al3+ as well as H+,
probably due to low content of
weatherable minerals (Yoshioka 1987, Patcharapreecha
et al. 1990, 1992) and (4) the Al3+ is partially bound with organic matter
(Patcharapreecha et al. 1990, 1992).

Accumulation
of salt (salinization) in the sandy soils

A rather large part of the arable land is salt-affected to varying degrees (Figure 4).
Generally, strongly salt-affected
narrow areas are scattered at the western hilly to undulating regions
and relatively weakly salt-affected wide areas are spread at the low-lying flat region, especially along the big
rivers (Figure 4). This suggests in most cases (1) the salt comes up to
the soil from a saline groundwater at localized
places in the hilly to undulating regions and (2) the uplifted salt spreads along the big rivers through the
groundwater. This suggestion is supported by the
fact that the groundwater is usually saline at these salt-affected
areas.

Figure 4. Soil salinity distribution (Arunin 1984)

Elevations inside the undulating region are conventionally classified into high, middle and
low terraces according to their relative height (Moorman et al. 1964). Inside the salt-affected area,
these terraces are differently
affected with salt. Salt affects only the foot of high terrace, from the top to the foot of middle terrace and
from the top till the foot of the low terrace. In addition, narrow low ground amongst these
terraces is often salt-affected.
Small salty mounds called nam dun in Thai are distributed mainly at the top of the middle terrace. Salt of nam dun appears to move
downward along the slope.

Based on these facts and other
information, the following tentative
theory was proposed for salinization in Northeast Thailand
(Khoyama and Subhasaram 1993, Wada et al. 1994).

The salt is originated from a
rock salt stratum in Mahasarakham
formation and comes up to the confined deep groundwater through deep fractures
formed at the boundary between the hilly and the undulating regions. The resulted saline confined deep groundwater
rises and passes the overlying clayey strata (mottled zone and pallid zone)
through their cracks developed mainly at the breast of middle terrace. The rising saline water (1) supplies
salt to the unconfined shallow
groundwater or (2) forms nam dun on
the ground surface. Nam
dun is enriched with not only salt but also clay and CaCO3, because the rising saline water is supplied with
these substances from the mottled zone and the pallid zone. Salt contained in nam dun moves downward along the slope by runoff,
interflow and baseflow in the rainy season. On the contrary, salt in the shallow groundwater salinizes the overlying soils by capillary rise in the dry
season.

Ameliorative technologies are somewhat different
between the non-saline soils and the salt-affected
soils, because the latter needs desalinization, an additional ameliorative technology. Accordingly, amelioration of the non-saline soil and the
salt-affected soil will be separately discussed.

Liming material: Liming is surely effective for
acid-susceptible crops. Liming the subsoil as well as to the surface soil is necessary if
the subsoil is acidified by heavy
application of chemical fertilizers (Ishida et al. 1993). However, the soil
with low buffering capacity against
pH-changes is liable to be over-limed and to induce deficiency of some
micronutrients (e.g. Zn, B). To keep the
neutralizing effect of liming for several years, coarse-grained CaCO3
is recom-mendable, because liming materials
are liable to be lost rather quickly from the sandy soil (Puengpan et
al. 1992, Ishida et al. 1993). For growth of
cotton in the acid soil, slaked lime + mixed chemical fertilizer was
much inferior to a city compost alone or in combination
with slaked lime and/or mixed chemical fertilizer
(Chairoj et al. 1993). Five years of successive applications of chemical
fertilizer resulted in poor growth of cotton due to rapid acidification of the
soil. On the contrary, 5 years successive compost application resulted in healthy growth of cotton (Ishida et al. 1993). These results indicate the
combination of coarse-grained CaCO3
with compost is most effective to overcome acidity of the sandy soil.

Chemical fertilizers: Many farmers cannot apply
sufficient amounts of chemical fertilizers to their crops, because yield of the crops is strongly
controlled by erratic rainfall. This
is especially so for paddy rice which is the main crop in Northeast Thailand. The problem
of chemical fertilizers to acidify the soil is discussed above. Another
problem of chemical N-fertilizer is its
low efficiency mainly due to loss of NH4+
and/or NO3- by leaching in upland field (Yoshioka
1987) and by lateral flow of floodwater (Chanchareonsook 1983), denitrification
and NH3-volatilization in the
paddy field. For finding methods to
prevent such loss, both the combination of chemical fertilizers with organic
matter (e.g. azolla) and the ball fertilizer
were tested using a lysimeter (Ishida et al. 1994). Both methods, especially latter, were effective in suppressing leaching of NH4+
and NO3-. The ball fertilizer is a ball shaped solid fertilizer composed of chemical fertilizers and a matrix to make
components of the chemical fertilizers release slowly to be efficiently
taken up by the plant. In an experiment, a
ball fertilizer of 4 mm in diameter was manually prepared by mixing chemical fertilizers with clayey material of the mottled or pallid zone rich in
2:1 type clay mineral. Accordingly,
such manually prepared ball fertilizer
may be recommendable for the poor farmers. In addition, ball fertilizer
including commercial products may be
recommended to the relatively rich farmers who are cultivating cash
crops by applying large amounts of chemical fertilizers along the waterway in the suburbs of big cities like Khon
Kaen, because excess amounts of chemical fertilizers are wasteful and potentially pollute water with NH4+,
NO3-and phosphate.

Clay: A pot experiment confirmed
addition of several kinds of clayey
material to the sandy soil increased growth
of sweet corn (Mitsuchi et al. 1986). One of the sources of the clayey material
examined in this experiment was the pallid zone rich in 2:1 type of clay mineral. This material was obtained from the
bank of a pond: the bank was
constructed using mainly the material
of the mottled zone and the pallid zone, which were dug out when the bottom of the pond reached the pallid zone. Farmers often cultivate cash crops
on the banks of their ponds.
Probably they recognize the soils of
the banks are better than the sandy soils of their fields. However, they will not bring the clayey
material from the pond to their remote fields for soil improvement, because it is too laborious. In this
sense, the ball fertilizer containing the clayey material mentioned above may be regarded as a practical
tool for applying the clayey material
to the sandy soil.

Compost and animal dung: Effectiveness of compost-application in increasing
rice yield and soil organic
matter content, mainly in the form of plant debris,
was confirmed by a long-term experiment in a paddy field at Surin (Takai 1983,
Saenjan et al. 1992). However,
compost-application is not traditional and the farmers have not accepted
official recommendation to use
compost. Probably, they dislike laborious work of preparation and application
of compost. Actually, raw material of compost is not abundant and quality of common compost is not high. For instance, most of rice stubble left in the
paddy field in large amounts is often burnt after being slightly grazed by cattle. Compost is usually prepared
outdoors under exposure to the rain
resulting in loss of water-soluble
substances such as K. Traditionally, the farmers use animal dung for cultivating cash crops and rice seedlings in
nursery beds. They may realize the effectiveness
of animal dung in promoting growth of the
crops and use the limited amount of animal dung as economically as possible. There are 2 reasons for the limited
amount of animal dung. The first is small number
of domestic animals. The second is that the farmers utilize only the dung excreted and stored under the floor of the farmers’ houses and will not
gather up the dung scattered
outdoors, probably they consider the weathered dung is ineffective.

Techniques to slow down the rate
of microbial decomposition
of the compost and the animal dung are necessary
for enhancing their utility, because the mineral
nutrients quickly released from them are liable to be lost in the same way as the chemical fertilizers before being
taken up by the crops. In addition, the techniques to slow down microbial decomposition of organic matter
in the soil should be necessary for increasing soil organic matter content and
for suppressing the increase in atmospheric
CO2 content.

For this purpose, addition of Al
or Fe salts to the compost and the animal
dung was expected to be promising, because
both metals have ability to strongly combine
with organic matter and retard its microbial decomposition. A laboratory
experiment confirmed these additives somewhat suppressed microbial decomposition of a buffalo dung, a city compost
and a rice straw compost and a field
experiment showed that the compost
added with these additives was better than the compost alone in enhancing
growth of baby corn in spite of that
these additives themselves were harmful to the crop (Saenjan et al.
1991, 1993). Furthermore, addition of
polyvinyl alcohol to the cow dung
appears effective also in suppressing microbial decomposition of the cow dung (Dejbhimon 2004).

Green manure: Utilization of
green manure has been
considered to be useful for increasing yield of crops and several plants have been recommended
for green manure.
For example, aquatic legumes have been regarded to be suitable for the rice cultivating in the rain-fed paddy fields, because
these plants can grow under
both drained and flooded conditions. However, farmers have not accepted the role of green
manures in their production systems. The reasons are (1) the seeds of recommended plants are not readily available, (2) chemical fertilizers should be applied to the
field to get sufficient amount of green manure, (3) many natural enemies
attack the recommended plants and (4) organic
acids toxic to rice seedlings are produced
when green manure is plowed under in
the flooded
paddy field. In addition, we must be
careful about the
problem of methane emission from the
flooded paddy
field when green manures are applied.
This additional
problem is especially serious for the sandy soil poor in
reactive iron oxide, because the
amount of methane
produced in the submerged soil
becomes high when the
soil is poor in reactive iron oxide
(Takai, 1961) and the
produced methane easily bubbles out from the soil
without oxidation at the oxidized layer by
methane
oxidizing bacteria (Chanchareonsook
et al., 1983; Taja,
1994).

Many experiments conducted in the
laboratory and in
the greenhouse (Patcharapreecha et al., 1993; Taja, 1994) revealed: (1) among aquatic legumes examined, Sesbania rostrata with stem-nodules as well as
root-nodules was the most promising plant for paddy
rice, because it can rapidly grow by actively fixing N2, (2) P was only one nutrient necessary for healthy
growth of the plant and phosphate rock could be used as a P-fertilizer,
(3) the amount of organic acids (e.g. acetic acid, butyric acid) produced from green manure in the submerged soil increased,
reached a maximum, decreased and
became very small 1 week after
incubation, (4) healthy growth of rice seedlings was secured when they were transplanted about 1 week after plowing under green manure in the
submerged, (5) methane was
actively produced within 2 weeks
after plowing under green manure in
the submerged
soil and (6) the methane production was remarkably
suppressed by placing green manure for about one
week on the surface of the submerged soil before
mixing with the submerged soil.

Furthermore, several field experiments
(Patcharapreecha et al. 1993, Sukchan 1994, Taja 1994) have shown: (1) drought, injurious nematodes and weeds as
well as P-deficiency were important factors limiting growth of S. rostrata in
the field though the harm of drought and nematodes were negligible and the weed-problem was not serious in the paddy
field, (2) S. rostrata could grow well in the moist upland field also and its growth was vigorous at the place temporarily flooded on occasion of heavy
rain, (3) toxicity of the organic
acids could be avoided by placing green manure on the ground surface of flooded paddy field for about 1 week before
plowing under in the soil, when bad
smell of butyric acid almost disappeared,
(4) phosphate rock was better than triple superphophate, a common P-fertilizer in Thailand, in the moist upland
field where the injurious nematode was active, (5) the recommended way
of green manure-application helped a farmer
to get rice yield (4.7 t ha-1), which was much higher than
average (Figure 7) and higher than the
estimated yield (4.2 t ha-1) of this cultivar, RD6; (6) soil fertility was improved, (7) the desirable effects of phosphate rock
continued in the paddy field for at least 3 years and (8) cattle grazed the tops of S. rostrata growing in the
drained paddy field resulting in
shortening of plant length, which was helpful
for harvesting and handling of the plant.

The combination of S. rostrata
and phosphate rock can be regarded as an in situ durable biological machine to produce P and N
available for common crops from the phosphate rock and the atmospheric molecular N2.
However, several problems still remain to
be solved for extending this combination to the farmers: For instance, it is necessary to establish seed systems of S. rostrata that ensure ease of
accessibility to farmers, to supply
phosphate rock of guaranteed quality to the farmers and to financially support
the farmers, who will adopt the
recommended techniques.

Management of the non-saline slope

In the undulating region, paddy fields are usually distributed on the gentle slope. In the
rainy season, flooding gradually
proceeds from the foot to the top of the slope. Accordingly, the paddy
fields covered with poor weeds remain idle
till flooding, even though absence of
pasture limits the number of cattle in Northeast Thailand.
A tentative plan to efficiently utilize this kind of the slope was proposed as shown in Figure 8
(Patcharapreecha et al. 1993).

In the beginning of rainy
season, all the paddy fields
and the neighbouring moist upland fields on the slope are applied with phosphate rock and planted
to S. rostrata. The plant growing in the paddy
field is cultivated
till the paddy field is sufficiently flooded and then used as green manure for paddy rice. The
plant growing in the upland fields is also utilized as green manure for upland crops at
appropriate time. During the period of cultivation of S. rostrata, the fields are used as
pastures for fattening cattle. This is desirable for crops also, because soil fertility is
increased with dropped dung and
application of green manure becomes easy due
to shortened plant length.

Amelioration of the salt-affected soil

As discussed above, degree and way of salinization
widely vary according to the position in the macro- and micro-relief. This
implies that desalinization technique should
be different according to the position in the relief and that lowering the
saline groundwater by reforestation, a widely accepted countermeasure of
salinization, should be carefully implemented
and/or supplemented by other counter- measures. At the same time, two aspects of desalinization strategy
should be considered. One is a short-term strategy for farmers and aims at
establishment of cheap, simple and profitable technologies suitable for
increasing yield of crops through improving
the soil of individual farmer’s field. The
other is a long-term strategy for the government and provides the government data and concepts for planning
approaches to combat salinization at the watershed level. These 2 strategies
should be well connected with each other so that the farmers and the government
work together for reclaiming the salt-affected area.

Short-term strategy for the farmer

Salt-affected slope in the undulating
region

Many experiments were conducted
at a slope of a middle terrace in the
salt-affected area (Puengpan et al. 1990,
Puengpan et al. 1991, Wada et al. 1993, Subhasaram 1994,). This is because almost all types of the salt-affected soil exist side by side on the
slope of the middle terrace. The
slope was a mosaic of the salt and vegetated patches. The salt patch is bare
and often covered with salt crust. Native weeds growing at the vegetated patch in the rainy season and those in
the dry season are different. The
former are annual and tolerant to wet
injury while the latter are perennial and tolerant to desiccation and salinity
(Puengpan et al. 1991). The salt
patch is often related with a dark colored layer about 10 cm in thickness developed near the ground surface. Depth of
the overlying sandy layer is thinner at the salt patch than at the vegetated
patch. The dark colored layer was
rich in both organic matter and clay and acted as an impermeable layer
in the rainy and a hard pan in the dry
season (Puengpan et al. 1990). In the rainy season, the dark colored
layer inhibited desalinization by leaching
and was strongly reduced. Thus, any native plants cannot grow at the
places where the dark colored layer is
present near the ground surface,
resulting in the salt patch. Actually, destruction of the dark colored layer was useful for plant
growth, especially at the weakly salt-affected places. The dark colored layer
may be resulted from deforestation near the top of the slope: the surface layer rich in organic matter and clay
of a former forest soil is selectively eroded
and the material rich in organic matter and clay is sediments at surrounding lower places of the slope. At the
foot of the slope, several paddy fields were abandoned due to salinization, which
was mainly caused by intrusion of salty mud
that came from the salinized slope by erosion and passed through broken dikes
(Puengpan et al. 1991).

On the one hand, rather many kinds of plant were cultivated for more than 1
year with alternating dry and rainy seasons at a wide salt patch on the slope for selecting plants to be used
for further experiments (Patcharapreecha et al. 1992, Puengpan et al. 1991). The selected plants were rhodes grass (Chloris gayana), Panicum repens, S. rostrata, S.
cannabina, Eucalyptus camaldulensis and Casualina sp. Rhodes
grass is a fodder plant and its seed is easily obtained from Live Stock
Experimental Station. Seedlings of Eucalyptus
camaldulensis are available in
the market.

On the other hand, cores were
vertically inserted into the ground
surface for examining factors con­trolling
desalinization and plant growth (Subhasaram et al. 1992, Subhasaram
1994). This experiment confirmed (1) the
soil inside the core was much more quickly desalinzed than the soil
outside the core, mainly due to enforced percolation of rainwater trapped
inside the core, (2) the bottom of the core should
reach the dark colored layer for avoiding lateral movement of water on
this layer, (3) mulch was effective in
suppressing accumulation of salt supplied by capillary rise of saline
water on sunny days, (4) germination of seed and growth
of a plant were possible inside the core
even at the salt patch, (5) application of cow dung enhanced
growth of the plant
if the degree if salinization was not too high and (6) the inserted core was ineffective in both
desalinization and germination at the place
with high
groundwater level, because the trapped rain water
remained stagnant inside the core.

A large-scale experiment was conducted at another wide salt patch to verify effectiveness of
4 soil-treatments (destruction of the dark colored layer, mulch, core-insertion and application of cow dung
or compost) on growth of 2 plants (rhodes grass and Panicum repens)
in the beginning of rainy season (Subhasaram et al. 1992, Subhasaram 1994). It
confirmed (1) each treatment was effective in promoting
growth of both plants, (2) cow dung was more effective in promoting
plant growth than compost, probably because
compost contained only small amount
of K, which was useful for plant growth in the salt-affected soil, (3) any combinations of these 4 treatments were more effective in plant growth
than each component treatment, (4) combination of the 4 treatments could
be selected according to characteristics of the site, (5) both plants can
vigorously grew even at the place with salt crust if 4 treatments were combined
together, (6) about 3 months after start of
the experiment, the wide salt patch
appeared fairly well covered with 2 plants and (7) the vegetated patch, especially rodes grass
patch, could be used as a pasture for
cattle.

Four treatments and their combinations were
collectively named “core technique”.

The
abandoned paddy field at the foot of the slope

The farmers will not repair the
broken dike of the
abandoned paddy field, though they recognize that runoff destroys the dike and
that the paddy field is damaged with the
salty mud intruding through the broken dike.
This is because the repaired dike, which is made of the dispersible
Na-saturated sandy soil, is easily broken by runoff. Accordingly, a dike
was prepared by a new simple method: plates
of cellocrete (synthetic concrete: Four Pattana Co., Thailand) were put
vertically at the center of a prepared dike (Subhasaram
1994). This new dike prevented runoff of salty mud entering the field, resulting in growth and yield of rice inside the abandoned paddy field.

Paddy field in the low-lying flat region

The sandy paddy field in the low-lying flat region often consists of the salt and vegetated
patches (Wada et al. 1994). Such paddy fields are usually underlain by thick clayey subsoil enriched with
salt. Salt comes up from the subsoil
to the sandy soil by capillary rise
in the dry season, leading to the salt patch at the place where the sandy surface soil is thin. This implies the cause of the salt patch here is
similar to that at the slope in the
undulating region mentioned above. However, in the low-lying flat region
with high groundwater level, the
technologies of desalinization established
in the undulating region should be modified in the following way: (Nagase 1992), mainly because destruction
of the saline thick clayey layer is impossible.

The paddy field was mulched in
the dry season and cultivated to S.
cannabina, which was more tolerant to
salinity than S. rostrata during initial period of the rainy
season. When the paddy field was sufficiently
flooded, S. cannabina was harvested and plowed under after
surface placement for about 1 week. Then,
rice seedlings were transplanted. This technique increased rice yield
about 3 times higher than average. Isolation of the salt patch seemed effective in assisting the healthy growth of rice
plant by inhibiting expansion of the
salt accumulated at the salt patch
to the whole paddy field through lateral flow of flood water.

Management of the salt-affected slope

Ameliorating technologies specific at each position of the slope should be consistent with
each other. From this standpoint,
the slope is divided into 4
sections: “upper section”, “salt-supplying section”, “erosion section” and “deposition
section” (Figure 9) (Subhasaram
1994). At “salt-supplying section”, salt is supplied from the confined
deep groundwater. Salt moves upward from
“salt-supplying section” to “upper section”
mainly by diffusion. In the rainy season, the salt of “salt-supplying section” together with dispersed mineral particle flows down the slope through
“erosion section” and deposits mainly at “deposition section”. In the dry season, some amount of salt is supplied
from the shallow groundwater by
capillary rise to the soil on the slope. This is especially evident at
“deposition section” with high level of the saline shallow groundwater. Thus, the whole slope is salinized,
though process of salinization
differs among 4 sections. The salinization is enhanced by deforestation,
because without forest, not only level of the saline shallow groundwater rises,
which favors the supply of salt from the
shallow groundwater by capillary rise, but also erosion of the salty mud
is accelerated by the strengthened runoff.

Reforestation has been widely accepted as a potent countermeasure to ameliorate the
salt-affected soil through lowering of the level of saline groundwater by transpiration of the tree.
Actually, in Northeast Thailand, several
places have been forested with eucalyptus, which has high ability of transpiration, for amelioration of the
salt-affected soils. However, these
eucalyptus forests did not desalinize the soil within a few years and often
dried up neighboring wells of villagers who are living principally at the non-saline area in the top of
middle terrace. In this case, the forest
may lower level the fresh shallow
groundwater, which supplies water to the wells, probably because the forest is too wide in size and is
too closely located to the wells.

Consequently, establishment of a “narrow forest” along the lower boundary of “upper
section” is proposed. The “narrow
forest” is expected to (1) inhibit migration of salt to “upper section”
from “salt-supplying section”, (2) lower level of the saline shallow groundwater and suppress runoff at
“erosion section” and (3) affect only slightly wells of the villagers.
Actually, few farmers planted eucalyptus along
the boundary between “upper section” and “salt-supplying section”. Probably, they have experienced such forest is effective to inhibit expansion of
the salt patch to “upper section”. One caution in planting eucalyptus is
that seedlings of the plant prefer the vegetated
patch to the salt patch. In other words, the salt patch should be improved to be the vegetated patch before planting eucalyptus (Puengpan et al.
1991). At “erosion section”, “core
technique” can be applied. At “deposition
section”, dikes of the paddy fields should be reinforced using the new
method (Subhasaram 1994), and should be managed according to the Nagase’s
method mentioned above. The boundary between
“erosion section” and “deposition section” is difficult to be managed, because
groundwater level is too high for
desalinization by “core technique” and too low for cultivating paddy rice. In Figure 9, planting of Cassurina sp. or halophytes is tentatively recommended in this area. Construction of beds with a thin coarse textured layer proposed by Sugi et al. was
expected to be useful for this
place, because surface of the bed is distant
from the groundwater by the height of the bed and the coarse textured layer inhibits capillary rise of the saline
groundwater (Takai et al. 1987). However, construction of the bed is laborious and dispersed fine soil particles quickly deteriorate the coarse
textured layer by filling its non-capillary pores. The surface layer aggregated with polyvinyl alcohol is found
to be better than the coarse
textured layer for desalinization at this place (Dejbhimon 2004).

Long-term strategy for the government

Among various governmental tasks for ameliorating the salt-affected soil, only one task
will be mentioned (Subhasaram 1994).

The farmer’s “narrow forest” is usually too narrow to play its all roles and is limited to the
field of each farmer concerned.
Government should make such incomplete
“narrow forest” wide and dense enough for playing its all roles
by paying due attention to the effect of
completed “narrow forest” on the wells of villagers and should connect
many fragmented farmer’s “narrow forests” to a continuous complete forest in
the whole salt-affected catchment. The farmers
may welcome this public work and agree to plant trees even in their arable fields if the government convinces the farmers of the intention and
significance of this public work.

A rather wide and dense
eucalyptus forest at the top of a middle terrace, which could be regarded as an example of the completed “narrow
forest”, has been shown
to lower the level of “shallow groundwater” year by year for a few years (Miura 1990). In spite of
this, salinity of the soils on the deforested slope of the middle terrace was
not much changed and growth of the plants
was inhibited during this short period. However,
10 years later, salinity of almost all the soils on the slope was
evidently decreased and plants including
paddy rice succeeded to grow. This may be caused by slow leaching out of the
salt accumulated in these soils due to lowered level of the shallow groundwater. This is one of the effects of the
long-term strategies (Subhasaram and
Wada 1999).

Concluding remarks

For achieving sustainable
management of the tropical sandy soil in
a region, it is imperative to understand
the properties of the soil and also natural and social conditions of the
region. This may help to understand the actual desires of the farmers
and to conceive ameliorative techniques
suitable for both the soil and the
farmers in the region. It is important to carefully examine advantages and disadvantages of all the conceivable
techniques in the laboratory, in the greenhouse and in the field. All results
of the examinations should be accessible to every person concerned including the farmers and the
governmental officers as well as the
scientists. In this context, some experiments
should be conducted at the farmers’ fields. Neighbouring farmers as well as owners of the fields will observe the
field experiments with great interest and adopt some techniques demonstrated in
the field experiments for managing their own fields. On the contrary, most of the farmers will not show any
interest in the field experiments
conducted in the Experimental Stations.
In addition, we must be careful about the fact that the soils inside Experimental Stations are often different from those of the farmer’s fields in
terms of fertility, though both of
them are identified as the same series
or same phase. The difference in the fertility is caused by difference in fertilization for many
years. The scientists may improve
the released technologies, which they are
interested in. The government may decide
policies based on the released technologies and concepts, which are desirable to both the farmers and the
government.

References

Arunin,
S. 1984. Characteristics and management of salt-affected
soils in the Northeast of Thailand. In: Ecology and Management of Problem Soils in Asia.
FFTC Book Series No. 27. Food and Fertilizer Technology Center for the Asian and Pacific Region. Taipei, Taiwan,
336-351.

Patcharapreecha,
P., Puengpan, N., Subhasaram, T. and Wada, H. 1992. Some characteristics of
plants growing at the salt-affected area in Northeast Thailand. Journal of the science society of Thailand, 18,
217-224.

Over the past 4 decades there has
been considerable expansion in the plantation forestry along the eastern seaboard of South Africa.
In particular there have been significant increases in eucalypt, and to a less
extent, pine plantations on soils
of a light sandy texture along the Zululand
coastal plain. These soils are characteristically
dominated by sands with low clay and organic matter contents, have low cation
exchange capacity and water holding
capacity. Pedogenesis and selected chemical attributes of a 49-year-old stand
of Eucalyptus grandis and Pinus elliottii established on these
sands were compared. Changes in soil pH, exchangeable cations, organic carbon, extractable Fe and Al and the
surface charge characteristics were investigated. Evidence of the development
of bleached A2e horizon within the surface 0-5 cm depth interval under E. grandis was confirmed through the
development of surface charge fingerprints, changes in organic carbon and Fe and Al mobilization for each of the
pedogenetically distinct horizons. Such development was not observed under the P. elliottii stand,
suggesting that this pine species has had less impact on the soil. It is argued that the rate of A2e horizon development is
not dissimilar to that observed under native forest ecosystems in Australia, although considerably
slower that those observed under reclaimed sand mining operations. Whilst these systems appear to be
relatively stable due to no clear felling and timber product extraction, this could drastically change with
the introduction of short-term rotations of fast growing clonal plantations,
questioning the long-term sustainability of these production systems on these
light textured sands.

Introduction

The role of vegetation in
processes associated with
pedogenesis is well recognised. In this respect the effects of tree species and
plantation forestry on soil properties have been the subject of numerous
studies and it has
been argued that specific plantation species reduce soil fertility, increase soil acidification and hence reduce productivity (Noble et al., 1996;
Routley and Routley 1975; Dasman 1972; Hamilton 1965; Khanna and Ulrich 1984). Many of these arguments were based on what was considered a parallel
situation in Europe,
where replacing broadleaved species with coniferous species (spruce) was
the cause of podsolized and infertile soils
(Turner and Kelly 1985). The establishment of eucalyptus plantations for
the production of pulp and
sawn timber has grown significantly over the past three decades in many countries other than Australia. This is in part due to
the rapid growth rates of this
species in environments that are devoid of natural predators, and more recently
the development of clonal forestry
with its associated high productivity and consistency of product. The impact of
eucalyptus and other species on the rapid development of podzols on sands
replaced after extensive mining of the
coastal sand dunes of eastern Australia
has been the subject of several
studies (Paton et al., 1976; Farmer et al., 1983; Thompson,
1992; Prosser and Roseby, 1995). These studies have clearly indicated
that the development of an A2 horizon is
rapid (4.5-5 years) and is often associated with the period when the greatest degree of leaching occurs (Prosser and
Roseby, 1995). In contrast, the establishment of Eucalyptus camaldulensis on mining spoils on the Jos Plateau of Nigeria had little effect on soil morphological attributes 15-20 years
after establishment (Alexander, 1989). The species significantly increased the amount of organic carbon with an
associated increase in cation exchange capacity (CEC). However, there
was a significant decline in soil pH and
base saturation and the author concludes
that the long-term effect of eucalypts is one of progressive degradation of already poor soils (Alexander, 1989).
Similarly, the leaching of soil columns
using the water soluble component extracted from the litter of eucalypts has been shown to lower the pH of
soils and mobilize both iron and aluminium (Bernhard-Reversat,
1999; Noble and Randall, 1999).

In the present study, we have
analyzed soils for changes in soil chemical properties under Eucalyptus grandis and Pinus
elliottii stands of similar age established on the same soil series on the Zululand coastal plain
of South Africa. Of particular interest in this study was the
quantification of changes in the surface
charge characteristics of soils collected from under both species.

Materials and method

Study site

The Langepan Correlated Curve
Trend (CCT) experiment
with Eucalyptus grandis and Pinus elliottii, was established in 1952 on a site
near KwaMbonambi (28º36′S; 32º13′E), KwaZulu
Natal, South Africa.
The experimental
site is situated on the coastal plain that extends
from Port Durnford in the south to the Mozambique border in the north and is at an altitude of 60 m a. s. l. The trial site is situated on
the boundary of the humid and
sub-humid zones of the summer rainfall
region of South Africa
(Bredenkamp, 1991). The mean annual rainfall is 1,400 mm, of which approximately 70% falls during the months of
October to April. The range in
temperature is relatively small due to the stabilizing influence of the
warm Mozambique current flowing down the eastern coast of South Africa.
Mean annual temperature, mean monthly maximum (January) and mean monthly
minimum (June) are 21.8ºC, 30.9ºC and 11.9ºC respectively (Bredenkamp, 1991).

The coastal plain is an elevated
marine platform that
consists essentially of a thick deposit of aeolian sand underlain by almost horizontal Cretaceous to Recent beds, dipping slightly seaward
(Bredenkamp, 1991). There are indications that the sands have been deposited
at intervals. The sands are acidic, of low fertility
and have poor horizon development. Due to wind transportation, the soils consist of medium sand (0.2-0.5 mm) grains with no coarse or fine sand
and very little silt or clay (0-6%).
These soils are generally poor in organic matter, due to rapid decomposition in
the moist subtropical climate and
the aerobic condition of the surface
soils. The water storage capacity of these soils
is very low, but this shortcoming is moderated by great depth. The soil was
classified as belonging to the Fernwood
series (ANON, 1991), a Dystric Regosol (FAO-UNESCO, 1990) or Quartzipsamment
(Soil Survey Staff, 1990). Adjacent to this C.C.T. trial, is a stand of Pinus elliottii that was
established at the same time as the
CCT trial on the same soil type.

In order to assess the impact of
the two species on soil chemical properties, two sites were selected in close proximity to the boundary
between the plantation systems. Soil pits were dug to a depth of 1.2 m with an exposed face of 2.5 m in each of the plantation
systems.

Soil analysis

Three soils samples were collected from the walls of
the pits in 2001, 49 years after the establishment of the CCT trial, from each pedologically distinct horizon. The samples were
air dried and sieved to pass a 2-mm
mesh before pH was measured in water
using a 1:5 soil:solution ratio. Basic exchangeable cations were
determined by atomic absorption spectrometry
after replacement with 0.1 M BaCl2/NH4Cl, as
recommended by Gillman and Sumpter (1986). Acidic cations (H+ + Al3+)
were extracted with 1 M KCl and the
extractant titrated to pH 8.0 as described by Rayment and Higginson (1992). The effective cation exchange capacity
(ECEC) was calculated as the sum of
basic and acidic cations (Ca2+
+ Mg2+ + K+ + Na+ + Al3+ + H+).
Soil organic carbon was determined by wet oxidation using the Walkley and Black method as modified by Rayment and Higginson (1992). In addition, soft
concretionary material (segregates) was collected from depth intervals in which they occur, air dried in the
same manner as the soil and ground to a fine powder for further
analysis. Charge fingerprints are curves describing
the total cation exchange capacity (CECT) and base cation
exchange capacity (CECB) across a range of pH values. They were
determined on composite samples from each
of the depth intervals using the methodology described by Gillman and Sumpter (1986). In brief, soils were Ca2+
saturated and brought to equilibrium
in a 0.002 M CaCl2 matrix. Suspension pH was adjusted to six values ranging from approximately 4.5 to 6.5. Once the desired range
of pH measurements had been achieved,
exchangeable Ca2+ and Al3+
were displaced with NH4 NO3. The Al3+ content in
solution was determined using the pyrocatechol-violet method (Bartlett et al., 1987). The amounts of Ca2+ and Al3+ adsorbed were
calculated taking into account the
amounts present in the entrained solutes. The
CECB is operationally defined as the Ca2+ adsorbed and CECT as the Ca2+
and Al3+ adsorbed. The pH buffer capacity of each layer collected was estimated from the amount of acid or base added
during the development of the surface
charge fingerprint. Linear regression plots
were constructed of amounts of acid/base added (mmolc H+/kg) versus pH. The inverse of the slope
of the regression curve was taken to
be indicative of the pH buffer
capacity (mmolc H+/kg.unit pH) of the soil.

Organic carbon (OC) was measured by dichromate
oxidation and spectrophotometric estimation
of residual dichromate on both the soil and soft segregate material (McLeod 1975). Organically complexed Fe and Al were extracted from 1 g of
soil and soft segregate material
with 100 mL of 0.1 M sodium pyrophosphate after overnight shaking
(Bascomb, 1968). Amorphous inorganic Fe and Al were extracted from 1 g of soil
and soft segregate material by 60 mL of 0.2 M ammonium oxalate adjusted to pH 3 and shaken in the dark for 4
hours (McKeague and Day 1966).
Aluminium and Fe were determined by atomic absorption spectrometry on the oxalate (Alox and Feox) and
pyrophosphate (Alpp and Fepp) extracts.

Results

Soil characteristics

The soil profile characteristics at each of the
sampled depth intervals for the E. grandis and P. elliottii plantation systems are presented in Table 1. Using the classification system of Isbell (1996),
the profile under the E. grandis stand
was classified as an Acidic
Regolithic Bleached-Leptic Tenosol whilst that under P. elliottii was classified as an Acidic Arenic Rudosol.
Under the eucalyptus stand there was a distinct O1 horizon that was made up of
organic materials at various stages of decomposition. The surface horizon (0-5 cm) was light brownish grey
when moist (10YR 6/2), however upon drying it exhibited a bleached (10YR
7/2) nature indicative of the development of a spodic horizon. Within this
layer a few (5%) soft organic segregations
with a diameter of 2-6 mm were
observed (Table 1). The horizon below (5-15 cm) became significantly
darker and showed signs of the development
of a Bhs horizon. The size and preponderance of soft organic
segregations increased to occupy
approximately 25% of the horizon. At depths below this horizon, the
presence of these organic
segregations dramatically declined so that at the 45-55 cm depth interval there was less that 2%
of the horizon that was occupied by these materials. In contrast to the profile under the
E. grandis stand, the P. elliottii profile was markedly different in that there was no evidence of the bleached spodic horizon development (Table 1). The presence of soft
organic segregations was evident
throughout the profile and occupied between <2% to 15% of any
individual horizon (Table 1). These soft
segregations left organic stains when crushed and wetted, with the
internal fabric containing particle sizes comparable with surrounding material in the layer, suggesting that
they were formed in situ.

Selected soil chemical
properties from each of the pits are presented in Table 2. Both profiles were acidic in reactivity with a mean
profile pH0.002 of 4.8 and 4.6 for E. grandis and P. elliottii respectively.
The acidic nature
of these profiles would account for the dominance of exchangeable acidity (Al3+ + H+)
on the exchange
complex over most basic cations. Within the surface horizons (0-15 cm) of the eucalyptus profile the dominant cation on the exchange
complex was Mg2+ with Ca2+ levels being significantly lower (Table 2). This trend was reversed under the
pine stand with Ca2+ being the dominant cation in the 0-22 cm depth
interval and Mg2+
being significantly lower. This may in part be due to Ca2+ lock up within the litter
layer (O1 horizon) present
under the eucalyptus stand. It is of note that the exchangeable K+ levels
in these soils were extremely low throughout the profiles of both species
suggesting that
this element may be limiting for optimal growth (Table 2). The effective cation exchange capacity (ECEC) of a soil is an indicative measure of the
cation exchange capacity at field
pH. In the E. grandis stand the ECEC ranged from 0.97 cmolc
kg-1 in the 5-15 cm depth
interval to a low of 0.50 cmolc kg-1 in the 45-55 cm depth interval. Contrasting this,
under the P. elliottii stand
the ECEC ranged from a high of 1.09 cmolc kg-1 in
the 0-10 cm depth and declined gradually
with depth to a low of 0.38 cmolc kg-1 in the 42-62
cm depth interval (Table 2).

The pH buffering capacity over all depth intervals was
highest under the pine species and declined
gradually with depth (Table 2). Contrasting this, the pH buffer capacity followed a similar trend as the ECEC under
the E. grandis stand with the highest buffering occurring in the
5-15 cm depth interval (Table 2). In general, the buffering capacity as
measured under both systems was low, suggesting limited internal resistance to
changes in pH. Soil organic carbon contents
for each depth interval and the segregations
collected from the profiles are presented in Table 3. Distinct
differences between the two plantation
systems were clearly evident with the E. grandis profile exhibiting an almost doubling (0.55%) of carbon content in the 5-15 cm depth interval
when compared to the horizons above
and below, indicating an accumulation of organic carbon in this depth (Table 3). The OC content in the 0-5 and 45-55 cm
remained constant at 0.27 and 0.26%
OC respectively. Contrasting this,
the OC content under the P. elliottii stand was highest (1.30%) in the 0-10 cm and declined sharply to 0.49% in the 10-22 cm depth to a low of
0.20% OC in the 42-62 cm depth interval (Table 3). These results clearly demonstrate the greater
amount of organic
carbon accumulation under the pine plantation when compared to the eucalypt.

Table
2. Selected soil chemical properties collected from pits in a long-term E.
grandis and P. elliottii stands. Values in parenthesis are the standard deviation from the mean

Depth (cm)

pHw

1pH0.002

EC

Na+

K+

Ca2+

Mg2+

Al2++H

ECEC

pH buffer capacity

E. grandis

(µS cm-1)

(cmole kg -1)

(mmolc
H+ kg1 . pH-1)

0-5

5.08

4.79

17.12 (0.35)

0.03 (0.00)

0.02 (0.00)

0.13 (0.01)

0.21 (0.02)

0.20 (0.01)

0.59 (0.04)

0.276

5-15

4.95

4.63

23.29 (0.38)

0.05 (0.00)

0.04 (0.00)

0.16 (0.01)

0.40 (0.03)

0.32 (0.01)

0.97 (0.03)

0.636

15-45

5.01

4.86

15.65 (0.39)

0.03 (0.00)

0.03 (0.00)

0.04 (0.00)

0.16 (0.01)

0.26 (0.00)

0.51 (0.01)

0.476

45-55

5.16

4.95

16.32 (1.89)

0.04 (0.00)

0.02 (0.00)

0.02 (0.00)

0.16 (0.01)

0.25 (0.01)

0.50 (0.01)

0.467

P. elliottii

0-10

4.79

4.42

16.21 (0.48)

0.01 (0.00)

0.02 (0.00)

0.63 (0.04)

0.14 (0.00)

0.29 (0.02)

1.09 (0.06)

1.066

10-22

4.73

4.61

12.91 (0.30)

0.01 (0.00)

0.01 (0.00)

0.25 (0.00)

0.05 (0.01)

0.33 (0.00)

0.66 (0.00)

0.878

22-42

4.68

4.77

9.99 (0.32)

0.01 (0.00)

0.01 (0.00)

0.08 (0.00)

0.04 (0.01)

0.29 (0.01)

0.43 (0.00)

0.602

42-62

4.75

4.85

9.95 (0.51)

0.01 (0.00)

0.01 (0.00)

0.05 (0.00)

0.03 (0.00)

0.28 (0.01)

0.38 (0.01)

0.586

1 pH0.002
pH measured in 0.002 M CaCl2 at the start of the equilibration
process in the development of the surface charge fingerprints.

2 CEC6.0
the CEC as pH 6.0 that was determined from the surface charge fingerprint.

The downward movement in the soil
profile of organic complexes of Fe and Al
as determined by pyrophosphate extractions has been least under P. elliottii when compared to the E. grandis (Table 3). Oxalate should
extract total translocated Fe and Al, including organic complexes extracted by
pyrophosphate (Farmer et al., 1983) although incomplete extraction of organic Al has been reported (Skjemstad et
al., 1992). Concentrations of Fepp, however are more than twice those of Feox in the case of
the E. grandis samples over all depth intervals (Table 3). In contrast, Fepp values were similar to Feox
in the 0-10 and 42-62 cm depth
intervals under the P. elliottii
stand suggesting that at these
depths the Fe is predominantly found
as an organic complex (Table 3). In
all depth intervals regardless of species, Fepp values were
considerably larger than Feox indicating the predominance of organic complex Fe. The fact that the subsoil matrix under each of the plantation
systems was little different in composition from the segregations suggests that these have formed in
situ, leaving small islands of clayey material that have become
hardened somewhat by Fe and Al oxides. However,
it is of note that in the cases of the E. grandis plantation the segregates showed much greater Fe,
Al and C accumulation suggesting that the effect of leaching solutions from the E. grandis litter
have been more drastic resulting in the move towards the development of a spodic horizon. Indirect
evidence for potential accelerated
podzolisation under E. grandis stand
can be implied from the greater propensity for the presence of
segregation material as outlined in Table
1. Clearly the degree of mobilization of both Fe and Al has been more intense under the E. grandis than
under the P.
elliottii stands respectively.

Surface charge fingerprints

By evaluating the charge
characteristics of these soils, a clear understanding of the impact of these two plantation systems on intrinsic
soil chemical properties can be assessed. The concept of charge fingerprinting as described by Gillman and
Sumpter (1986) provides an assessment of both the positive and negative charge characteristics of a soil over a pH range that has significance when assessing the impact of
plantation systems on the soils resource. When used in conjunction with exchangeable cations extracted
from the exchange complex, an
assessment of current and potential
nutrient-holding capacity and the impact of management can be assessed.

The methodology used to develop
the charge fingerprint
estimates the CECB and CECT at each pH point. The CECB is the
total amount of basic cations that can be retained in an exchangeable form at any particular solution pH and ionic strength. The total cation exchange capacity (CECT) is the
total amount of basic and acidic cations that can be retained in an exchangeable form at any particular solution pH
and ionic strength. The approach distinguishes
that portion of the cation exchange capacity (CEC) that retains basic
cations, and predicts changes in CEC as soil solution
pH and ionic strength are varied. For brevity only the CECT
is discussed.

These soils are dominated by sand with very little clay. Consequently, the surface charge
generation potential associated with changes in pH is limited. Figure 1
shows charge fingerprints for composite samples collected from
the 0-5, 5-15, 15-45 and 45-55 cm depth intervals for the E. grandis soil.
A distinct characteristics of the curve derived for the 0-5 cm depth interval
is the quantity of negative charges
generated, namely 0.24 cmolc kg-1, over the pH range 4.5 to 6.5 (Figure 1 and Table 3).
Contrasting this, in the 5-15 cm
depth interval the amount of charge generated over the same pH range
trebled to 0.846 cmolc kg-1,
clearly indicating the influence of accumulated
organic carbon or remaining carbon in this depth interval (Figure 1 and Table
3). Over the remaining depth intervals, the amount of charge generated
over the pH range 4.5 to 6.5 remained relatively constant with values of 0.612
and 0.722 cmolc kg-1 respectively (Figure 1 and Table 3).
In contrast, under the P. elliottii plantation
the greatest amount of charge
generated over the pH range 4.5 to 6.5
was 1.644 cmolc kg-1 in the surface 0-10 cm depth
interval and declined progressively down the profile to a value of 0.420
cmolc kg-1 in the 42-62 cm depth interval corresponding
to changes in soil organic carbon (Figure 1
and Table 3). The greatest difference in
the shapes of the charge curves was in the surface horizons of the two
plantation systems. This can be ascribed
to the larger organic carbon content under the P. elliottii plantation when compared to the E.
grandis and
clearly quantifies the potentially deleterious impact of this species on exchange properties on soils with a small permanent charge. In short, the role of
organic C in maintaining negative charge on these soils is critical for
the retention of cations. In addition, an evaluation
of the surface charge characteristics of these samples clearly indicates
the position in the profile where the
development of a spodic horizon (5-15 cm) has occurred under the E. grandis and
quantifies the influence of these processes on the surface charge
characteristics of these soils.

If the basic and acidic cations
removed by the BaCl2-NH4Cl
and KCl extractants, respectively, are all exchangeable
cations, then their sum (the ECEC) should
be equal to CECT at soil pH, within the limits of the experimental
error. A graph of ECEC against CECT
at the soil’s pH (Figure 2) for the two plantation species shows good
agreement between these independently
determined properties for the surface samples.
However, with depth there was less cations extracted than could be accounted for by CECT. This would
suggest that cations that are present on the exchange
complex are not accounted for in the BaCl2-NH4Cl and KCl extractants. A possible
cation that may have contributed to
an underestimation of the ECEC could be Fe. Indeed, as a significant amount of
oxalate and pyrophosphate Fe was
extracted from the soils, some may have been associated with the
exchange complex (Table 3).

Figure 2. Relationship
between total cation exchange capacity at soil pH and the effective cation
exchange capacity (ECEC) for each of the
depth intervals sampled. The line
represents the 1:1 relationship between CEC and ECEC. The values falling close to the line are for the surface
samples, E. grandis (0-5 cm) and P. elliottii (0-10 cm)

Discussion and Conclusions

Analysis
of the soil pits assumes that differences between sites are due to the direct
influence of the plantation species and that variations in parent material,
topography and other factors are relatively unimportant. As the area has been
extensively planted to eucalyptus and pines species, undisturbed or pristine
sites containing native vegetation components could not be sampled as a
control. Consequently it is assumed that at the time of establishment of these
two production systems soil attributes were similar. Assuming that this was the
case, an assessment of the chemical and morphological properties of soil
profiles under each of the production systems clearly indicates that there have
been considerable changes associated with the tree species. There is clear
evidence that under E. grandis the early stages of a bleached spodic
(A2e) horizon development is clearly evident in the 0-5 cm depth interval. In
addition, constructing surface charge fingerprints confirms the presence of the
spodic horizon and the development of a rudimentary Bhs horizon associated with
the accumulation of organic complexes in the 5-15 cm depth. Such morphological
and chemical changes in soil properties were not evident under the P.
elliottii stand.

It is
important to note that these two systems have had very little disturbance
associated with traffic movement within the plantation. This has undoubted
allowed the effective observation of horizon development from the surface to
depth. This would not be the case in plantations that have had mechanical
traffic through the plantation that would disturb surface soil horizons thereby
homogenizing the soil making the delineation of a rudimentary A2e horizon
difficult.

Studies
into the development of podzols on the east coast of Australia have shown that thousands
of years are required to develop mature profiles. For example, giant podzols
with A2 horizons 12 to 22 m thick have formed over periods of up to 700,000
years (Tejan-Kella et al., 1990). At the younger end of the scale, the depth to
the B horizon can be 1.6 m or less on Holocene dunes and less than 50 cm on
dunes deposited over the last 3,000 years (Pye, 1981; Thom et al., 1981;
Thompson, 1983; Bowman, 1989). Contrasting this, Prosser et al., (1995)
reported the development of an A2 horizon to a depth of at least 3.7 m to have
formed within 17 years on post mined sand dunes. In the present study the depth
of the rudimentary A2 horizon was a mere 5 cm after 49 years. This rate of
development is approximately 10 times faster than those reported above for
Holocene dunes in Australia but
considerable slower than that by Prosser et al. (1995). Prosser et
al. (1995) attributed this unprecedented rate of pedogenesis to the high
permeability of the sands, the low silt and clay content, the previous advanced
stage of weathering and pedogenesis, and the homogenization of the soil during
mining operations. Whilst the current study would suggest that the rate of
development of an A2 horizon is not drastically dissimilar to natural systems,
it is prudent to note that the stand had never been felled and hence would
represent effectively a ‘climax’ stand; the leaching component under this
system would be very small, thereby reducing the rate of A2 development; and
most importantly, as these systems had not undergone any form of surface
disturbance it allowed us to identify the presence of an A2 horizon. In the
current climate of moving to short rotations (4-8 years) using clonal material
that place a significant demand on soil and water resources including whole
tree harvesting and potential for greater leaching to occur due to the reduced
rotation length, the potential negative impact of such forestry systems on soil
resources that have limited intrinsic attributes is great. The impact of P.
elliottii under the prevailing circumstances would appear to be minimal
when compared to other species and would support previously reported studies
(Noble et al., 1999). Finally, the development of surface charge
fingerprints has demonstrated the usefulness of this technique in quantifying
the influence of pedogenesis on intrinsic soil properties and could be a
potential tool in assessing horizon development at an early stage.

References

Alexander, M.J. 1989. The long-term effect of
Eucalyptus plantations on tin-mine spoil and its implication for reclamation. Landscape
and Urban Planning. 17: 47-60.

ANON. 1991. Soil classification a taxonomic system for South Africa.
Published by the Directorate Agricultural Information: Pretoria, South Africa.

Routley, R., and Routley, V. 1975. The fight for the
forests. The takeover of Australian forests for pinewood chips and intensive
forestry. Research School of Social Services. Australia National
University, Canberra.